Fuel cell stacks of alternating polarity membrane electrode assemblies

ABSTRACT

Improved water distribution can be obtained within the cells of a fuel cell series stack by maintaining a suitable temperature difference between the cathode and anode sides of each cell in the stack during shutdown.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods and designs for obtaining improvedwater distribution within the cells of a fuel cell series stack duringshutdown and, more particularly, to the shutdown of solid polymerelectrolyte fuel cell stacks.

2. Description of the Prior Art

Fuel cell systems are presently being developed for use as powersupplies in a wide variety of applications, such as stationary powerplants and portable power units. Such systems offer the promise ofeconomically delivering power while providing environmental benefits.

Fuel cells convert fuel and oxidant reactants to generate electric powerand reaction products. They generally employ an electrolyte disposedbetween cathode and anode electrodes. A catalyst typically induces thedesired electrochemical reactions at the electrodes.

A preferred fuel cell type, particularly for portable and motiveapplications, is the solid polymer electrolyte (SPE) fuel cell whichcomprises a solid polymer electrolyte membrane and operates atrelatively low temperatures.

SPE fuel cells employ a membrane electrode assembly (MEA) whichcomprises the solid polymer electrolyte or ion-exchange membranedisposed between the cathode and anode. Each electrode contains acatalyst layer, comprising an appropriate catalyst, located next to thesolid polymer electrolyte membrane. The catalyst is typically a preciousmetal composition (e.g., platinum metal black or an alloy thereof) andmay be provided on a suitable support (e.g., fine platinum particlessupported on a carbon black support). The catalyst layers may contain anionomer similar to that used for the solid polymer electrolyte membrane(e.g., Nafion®). The electrodes may also contain a porous, electricallyconductive substrate that may be employed for purposes of mechanicalsupport, electrical conduction, and/or reactant distribution, thusserving as a fluid diffusion layer. Flow field plates for directing thereactants across one surface of each electrode or electrode substrate,are disposed on each side of the MEA. In operation, the output voltageof an individual fuel cell under load is generally below one volt.Therefore, in order to provide greater output voltage, numerous cellsare usually stacked together and are connected in series to create ahigher voltage fuel cell series stack.

During normal operation of a SPE fuel cell, fuel is electrochemicallyoxidized at the anode catalyst, typically resulting in the generation ofprotons, electrons, and possibly other species depending on the fuelemployed. The protons are conducted from the reaction sites at whichthey are generated, through the electrolyte, to electrochemically reactwith the oxidant at the cathode catalyst. The electrons travel throughan external circuit providing useable power and then react with theprotons and oxidant at the cathode catalyst to generate water reactionproduct.

In some fuel cell applications, the demand for power can essentially becontinuous and thus the stack may rarely be shutdown (such as formaintenance). However, in many applications (e.g., automotive), a fuelcell stack may frequently be stopped and restarted with significantstorage periods in between. Such cyclic use can pose certain problems inSPE fuel cell stacks. For instance, in U.S. Patent ApplicationPublication Nos. US 2002/0076582 and US 2002/0076583, it is disclosedhow conditions leading to cathode corrosion can arise during startup andshutdown and that corrosion may be reduced by rapidly purging the anodeflow field with an appropriate fluid.

Other problems that can arise from cyclic use relate to the watercontent remaining and its distribution in the stack after shutdown. Forinstance, liquid water accumulations in the stack can result from toomuch water remaining and/or undesirable distribution during shutdown.Such accumulations of liquid water can adversely affect cell performanceby blocking the flow of reactants and/or by-products. Perhaps evenworse, if the fuel cell stack is stored at below freezing temperatures,liquid water accumulations in the cells can freeze and possibly resultin permanent damage to the cells. On the other hand, with too littlewater remaining, the conductivity of the membrane electrolyte can besubstantially reduced, with resulting poor power capability from thestack when restarting.

Given these difficulties, there remains a need in the art to developprocedures and/or design modifications in order to obtain improved waterdistribution in fuel cell stacks during shutdown and storage. Thepresent invention addresses these and other needs, and provides furtherrelated advantages.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that a desirable water distribution can beobtained in a fuel cell series stack after shutdown by ensuring that anappropriate temperature difference is maintained across the cells in thestack as it cools during the shutdown. In this way, for instance, theremaining water in a solid polymer electrolyte fuel cell stack can beconcentrated in a selected set of colder flow fields and dealt withappropriately, while still maintaining sufficient water in the membraneelectrolyte for purposes of conductivity.

Specifically, the method of the invention applies to a fuel cell seriesstack which has at least two cells and typically a plurality of cellsstacked in series. The method is particularly suitable for solid polymerelectrolyte fuel cell series stacks. The shutdown method comprisesstopping the generation of electricity from the stack, and allowing thestack to cool in a controlled manner over a certain period (i.e., acooling period) in which a temperature difference is maintained betweenthe cathode and anode sides of each cell in the stack and the directionof the temperature difference in each of the cells is the same. That is,either the cathode is hotter than the anode in each cell during thecooldown period or vice versa. With this approach, the water in eachcell generally migrates to the colder side during cooldown.

A suitable shutdown method involves maintaining substantially the sameabsolute temperatures and temperature difference within each cell duringthe cooldown period (e.g., the cathode side temperature in each cell isabout the same and the anode side temperature in each cell is about thesame). The resulting temperature profile between the two ends of thestack will then resemble a sawtooth shape with each cell temperatureprofile corresponding to a tooth. Such a profile may be obtained bythermally insulating each cell from adjacent fuel cells in the stack(e.g., by increasing the thermal resistance between each pair of cells)and by modestly cooling a selected set of electrodes (e.g., cooling aset of electrodes immediately adjacent to coolant channels in thestack).

An alternative shutdown method involves maintaining a monotonicallydecreasing temperature across groups of fuel cells in the stack duringthe cooldown period. That is, each group of fuel cells has a hot sideand a cold side, the temperatures of the fuel cells in each groupdecrease monotonically across the group between the hot side and thecold side during the cooldown period, and the temperatures of, and thetemperature difference between, the hot side and the cold side of eachgroup during the cooldown period are substantially the same. Again, theresulting temperature profile between the two ends of the stack willresemble a sawtooth shape although here, the temperature profile of agroup of cells corresponds to a tooth. Such a profile may be obtainedfor instance by incorporating Peltier devices between groups of cells.Each Peltier device serves to cool the “cold” side of one cell groupwhile it heats the “hot” side of the adjacent cell group.

Yet another alternative shutdown method involves maintaining amonotonically decreasing temperature across the entire stack during thecooldown period. Such a profile may be obtained by heating the “hot” endof the stack or perhaps merely by thermally insulating the “hot” end ofthe stack in order to keep it sufficiently hot. Alternatively, such aprofile may be obtained by suitably cooling the “cold” end of the stack.

A typical stack may be at a temperature greater than about 70° C. priorto shutdown and thus prior to the cooldown period. The cooldown periodmay need to last until the stack temperature is less than about 40° C.at the colder end. An effective temperature difference during thecooling period is about or just greater than 1° C. per cell and aneffective cooldown period can be about or greater than about 20 minutes.However, smaller temperature differences and/or shorter cooldown timescan also be expected to be effective.

The fuel cells in a typical SPE stack comprise cathode and anodereactant flow fields and the method can allow for water to beconcentrated in either set of flow fields (e.g., cathode side) duringshutdown. Depending on the specific embodiment and operating conditions,these reactant flow fields may or may not desirably be purged during thecooldown period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a solid polymer electrolyte fuelcell series stack and the direction of water flow within the cells whensubjected to a temperature difference.

FIG. 2 a shows the water transmission characteristics through a MEA as afunction of time with two different temperature differences across theMEA.

FIG. 2 b shows the water transmission characteristics through a MEA as afunction of temperature with a constant temperature difference acrossthe MEA.

FIG. 3 a shows the temperature profile across the fuel cell stack ofExample 4 when cooled down according to a method of the invention.

FIG. 3 b shows the temperature profile across the fuel cell stack ofExample 4 when cooled down in a conventional manner.

DETAILED DESCRIPTION OF THE INVENTION

The shutdown method of the invention is particularly useful forachieving a desired water distribution in a SPE fuel cell stack. Anexemplary SPE fuel cell stack is shown schematically in FIG. 1. Stack 1comprises a plurality of stacked cells including end cells 2 and 3 atthe negative and positive ends of stack 1, respectively. In between area plurality of cells 4 (for simplicity only two in-between cells aredepicted in FIG. 1). Each cell comprises a solid polymer electrolytemembrane 5. Suitable catalyst layers (not shown) serve as the anode andcathode in each cell and are applied to opposing faces of each membrane5. Each cell also comprises an anode gas diffusion layer 6 and a cathodegas diffusion layer 7. And, adjacent to the gas diffusion layers 6 and 7in each cell are an anode flow field plate 8 and a cathode flow fieldplate 9, respectively. Each plate comprises anode flow field channels 10and cathode flow field channels 11, respectively. As depicted, eachanode flow field plate 8 (other than that in end cell 2) also containscoolant flow field channels 12. Typically, negative and positive busplates (not shown) and a pair of compression plates (not shown) are alsoprovided at either end of the stack. Fluids are supplied to and from thereactant and coolant flow fields via various ports and manifolds (notshown).

In the method of the invention, a temperature difference is maintainedbetween the anode and cathode side of each cell in stack 1 over acooldown period when the stack is shutdown. In FIG. 1, the temperaturedepicted decreases from anode to cathode. The exposure to such adifference over a suitable period of time results in a significantmovement of water within each cell (via a distillation and condensationmechanism), with the transfer going from the anode to the cathode side.With such a temperature difference, most of the transferred watercollects in the cathode flow fields 11, although enough water can remainin the electrolyte membranes 5 to keep the ionic conductivity at asatisfactory level. Once the cooldown period is over, the stack istypically allowed to equilibrate to ambient temperature.

As illustrated in FIG. 1 and in the following Examples, water iscollected in the cathode flow fields of the SPE fuel cell stacks duringshutdown. This is desirable when the stack design is such that the watercollected at the cathode side can be accommodated without blocking thecathode flow fields and when the collected water isn't a problem withregard to freezing during storage. It may also be desired if the cathodeflow field is readily drained or is readily purged via some appropriatemeans (e.g., such as with an inert gas) during shutdown, therebyremoving the collected water. However, due to differences inconstruction and/or operation (e.g., if anode purging were employedduring shutdown), it may instead be desired in other stack embodimentsto direct the water to the anode side during shutdown, thus making anopposite direction for the temperature difference preferred.

In conventional fuel cell stacks, the stack ends typically cool somewhatfaster than the remainder of the stack and thus the temperature profileacross the stack during cooldown is not monotonic nor is the temperaturedifference across each cell in the same direction. Thus, to achieve thedesired temperature difference, either the stack temperature must becontrolled appropriately during shutdown or modifications must be madeto the cell and/or stack structures.

In one embodiment, a similar temperature profile may be established overeach cell during the cooldown period (i.e., each anode flow field platetemperature is about the same and each cathode flow field platetemperature is similar). This may be achieved for instance by using thecoolant in coolant channels 12 to cool each cathode flow field plate 9slightly more than each anode flow field plate 8 during shutdown. Thetemperature of a representative “hotter” part of the stack may bemonitored and used to control and ramp down the temperature of theflowing coolant appropriately such that the anode flow field plates 8are always just warmer than the cathode flow field plates 9.

In order for a desired temperature difference to exist between anodeflow field channels 10 and adjacent cathode flow field channels 11, itmay be necessary to increase the thermal resistance between each pair ofcells. In FIG. 1, the thermal resistance might be increased by modifyingthe anode flow field plates 8 in planar regions 13 which separate theanode flow field channels 10 from coolant channels 12. The modificationsmay include increasing the separation of channels 10 from channels 12,or introducing insulating voids in regions 13, or employing differentmaterials in regions 13 with greater thermal resistance than that in therest of plates 8. Alternatively, the thermal resistance might beincreased by lining a side of coolant channels 12 with thermalinsulating liners 14 made of a material with greater thermal resistancethan that in the rest of plates 8. Either way, the coolant in coolantchannels 12 would more readily cool cathode flow field channels 11 thananode flow field channels 10 thus causing water to collect in theformer.

In another embodiment, a monotonically decreasing temperature ismaintained across groups of cells in the stack during the cooldownperiod. Here, each group has a hot and cold side and the temperaturedecreases monotonically from the hot side to the cold side. Groups ofcells that can be cooled in this manner may be created by incorporatingPeltier devices between the desired groups. The Peltier device 15depicted in FIG. 1 for instance defines two groups of cells, an upperpair and a lower pair. Peltier device 15 cools the cathode flow fieldplate 9 immediately above it and heats the anode flow field plate 8immediately below it.

In yet another embodiment, a monotonically decreasing temperature ismaintained across the entire stack during the cooldown period. Such atemperature profile may be obtained in a stack equipped with a heater atthe desired “hot” end (e.g., at anode flow field plate 8 in end cell 2)by controlling the heater output appropriately while the other end ofthe stack (e.g., end cell 3) naturally cools when power productionstops. A suitable starting temperature difference can be established inthis manner. Then the stack can be cooled so as to maintain amonotonically decreasing temperature across it by ramping down thetemperature of the heater appropriately. It may be useful to monitor thestack temperature at one or more points and use this information tocontrol the heater output. In other embodiments, thermal insulationalone might substitute for the heater at the “hot” end. During cooldown,the insulation would have to be sufficient to retain enough heat so asto establish the desired difference while the other end of the stacknaturally cooled. Further still, cooling at the “cold” end may also becontrolled (e.g., via Peltier devices or by flowing coolant).

In selecting which of the above or other possible embodiments to use ina given application, consideration should be given to the structure ofthe fuel cell stack and its normal operating conditions. For instance,temperature differences of the order of 1° C. per cell may be effectivein distributing the water within the cells in reasonable timeframes(e.g., minutes). In stacks having only a few cells, maintaining amonotonically decreasing temperature over the entire stack duringshutdown may then be a preferred option. However, this approach isimpractical for stacks comprising more than a hundred cells. In thiscase, the temperature difference required across the entire stack wouldthen have to be of order of 100° C., a value perhaps greater than thetypical stack operating temperature. Thus here, an alternative approachmay need to be selected instead (e.g., applying a similar absolutetemperature profile across each cell during shutdown). The direction ofthe temperature difference during the cooldown period is of courseselected according to where liquid water is preferably collected duringshutdown (e.g., either cathode or anode flow field channels). This watermay desirably be purged from the stack as part of the shutdownprocedure. The duration of the cooldown period and cooling rate arechosen such that water has sufficient opportunity to migrate within thecell. Those skilled in the art can be expected to select an appropriatemethod and cell/stack modifications to suit a given application.

The following examples are provided to illustrate certain aspects andembodiments of the invention but should not be construed as limiting inany way.

EXAMPLES Example 1

Two high aspect ratio SPE fuel cells were made in which the MEAscomprised NAFION® N112 perfluorosulfonic acid membrane electrolyte withcarbon supported Pt/Ru catalyst applied on one face and carbon supportedPt catalyst on the other face to serve as anode and cathode electrodesrespectively. The MEAs also comprised polytetrafluoroethylene (PTFE)impregnated carbon fibre paper substrates to serve as gas diffusionlayers on each side of the catalyst coated membrane electrolyte.Grafoil® graphite reactant flow field plates with linear flow channelsformed therein were located on either side of the MEAs, therebycompleting the fuel cell assembly.

For testing purposes, independently controllable electrically heated busplates were placed on each side and the cells were initially operated onhydrogen and air reactants at 70° C. Then the flow resistance of thehydrogen fuel in the anode flow field plate was measured as a functionof temperature difference across the cells. The temperature differencewas varied from −5° C. (anode colder than cathode) to +8° C. (anodewarmer than cathode) by varying the cathode side heater appropriatelywhile leaving the anode side temperature constant. Table 1 below showsthe results from the three cells tested. TABLE 1 Temperature Flow FlowFlow of anode resistance resistance resistance relative to (Kpa/SLPM)(Kpa/SLPM) (Kpa/SLPM) cathode (° C.) cell 1 cell 2 cell 3 +8 12 10 14 −215 13 18 −3 15 20 38 −5 27 27 NA

As is apparent from Table 1, the anode flow resistance was relativelylow and acceptable when the anode was hotter than the cathode. However,when the anode was colder than the cathode, the flow resistance variedsignificantly and was generally undesirably high. It is believed thatthe increase in flow resistance results from liquid water collecting inthe anode flow fields. This Example illustrates how obtaining a desiredwater distribution (in which blockage of the anode flow field isavoided) may be affected by the temperature difference across the MEA.

Example 2

A fuel cell assembly similar to those of Example 1 was prepared in whichabout 6 g of water was applied to the cathode flow field plate but theanode side was dry. The membrane electrolyte contained about 5% absorbedwater by weight (equilibrium level at room temperature and humidityconditions). The wet cathode side of the cell assembly was then placedon a hot plate set at 70° C. and the anode side insulated such that theanode flow field plate was about 2° C. colder than the cathode flowfield plate. In this arrangement, water transfers from the cathode sideto the anode side of the cell and accumulates at the anode plate. Therate of water transfer from the cathode to the anode side was thendetermined via weight gain measurements of the anode plate. (The cellwas removed and allowed to cool for about 30 seconds before each weightmeasurement of the anode plate). The test was repeated without usinginsulation on the anode side such that now the anode flow field platewas about 4° C. colder than the cathode flow field plate.

FIG. 2 a shows the rate of water transfer through the MEA as a functionof time with these two different temperature differences across the MEA.In both cases, a roughly constant rate of water transfer is observedsuggesting that water transfer is driven by a concentration gradient.

A fuel cell assembly similar to those of Example 1 was again prepared inwhich water was applied to the cathode flow field plate and the anodeside was dry. This time, the rate of water transfer was measured after 1minute of heating at different cell temperatures using the electricalheating apparatus of Example 1. Here, the cathode flow field plate waskept hotter than the anode by a constant 5° C. Again, weight gain of theanode plate was determined. (To avoid water loss from evaporation at thehigher temperatures here, the anode plate was quickly clamped to acondenser plate and both plates were weighed.)

FIG. 2 b shows the rate of water transmission in g per ° C. through theMEA as a function of temperature. The temperature of the cathode plateis plotted on the X axis. FIG. 2 b also shows the water saturationpressure as a function of temperature. Both plots show a similar shape.With increasing temperature, both the concentration gradient and therate of diffusion for water through the membrane increase.

This Example provides some quantitative information about water transfercharacteristics through conventional MEAs.

Example 3

A fuel cell assembly similar to that of Example 1 was prepared with twocircular pieces cut out of the MEA. The MEA was wetted and the weight ofthe gas diffusion layers and the catalyst coated membrane in thesecutout pieces was determined. The weights of a wetted anode flow fieldplate and a dry cathode flow field plate were also determined and a fuelcell assembly was prepared by placing the MEA (including the cutoutcircular pieces) between the plates. A temperature difference of about2-3° C. was applied across the fuel cell assembly (with anode hotter)for about 20 minutes at 60° C. using the heating apparatus of Example 1.Thereafter, the fuel cell assembly was disassembled and the weights ofthe various components measured again. The weight of water in eachcomponent before and after exposure to the temperature difference wasthen calculated by subtracting the known dry weight of each component.The results appear in Table 2. TABLE 2 Anode Cathode Cathode gasCatalyst Anode gas flow flow field diffusion coated diffusion fieldplate layer membrane layer plate Weight of 0 0.88 0.55 0.08 2.63 waterbefore (g) Weight of 2.77 −0.01 0.3 −0.02 0 water after (g)(The slight negative values appearing in Table 2 result fromexperimental error.)

Exposure to the temperature difference has caused the water at the anodeflow field plate to migrate to the cathode flow field plate. The anodeflow field plate and anode and cathode gas diffusion layers arecompletely dry. Purging with a dry gas was not required to dry thesecomponents. However, the catalyst coated membrane still retained anamount of water that is expected to provide sufficient membraneconductivity for starting up the fuel cell. (Those skilled in the artwill appreciate that greater membrane hydration may be desired, and thusengineered in on shutdown, depending on the desired startup capabilityof the fuel cell stack.)

This example demonstrates that application of a modest temperaturedifference can significantly move the water within a cell to desirablelocations.

Example 4

A series stack was prepared using 30 fuel cells similar to those inExample 1. In each cell, the anode flow field plates were wetted withabout 3-4 g of water prior to assembly. Thermal insulation (foam) waswrapped all around the stack and independently controllable electricallyheated bus plates were placed on each end of the stack. The stack wasthen used to compare water distribution results after an inventivecooldown method to those after a conventional cooldown procedure.

A cooldown method illustrative of the invention was achieved by heatingthe anode end of the stack to 80° C. and the cathode end to 50° C. usingthe heaters. Once equilibrium was obtained, the cathode side heater wasturned off to improve the temperature difference across the stack. Then,the cooldown simply involved reducing the anode side temperaturegradually with time. FIG. 3 a shows the temperature profile across thefuel cell stack at various times during the cooldown. Here, amonotonically decreasing temperature is maintained across the stackduring the cooldown period. When the stack had cooled to about 30° C.,it was disassembled and analyzed for water distribution. The analysesinvolved: (1) visually estimating the amount of water in the anode andcathode flow field plates; (2) measuring the weight of water in thewhole MEAs; and (3) measuring the weight of water in sample MEAcomponents (obtained from several circular pieces cut from the MEA as inExample 3). For comparison purposes, the stack was re-made as above butthis time was heated up to 80° C. at both the cathode and anode ends.Once equilibrium was obtained, the heaters were simply turned off.Thereafter, a conventional cooldown followed as the stack lost heatnaturally to the surrounding environment. FIG. 3 b shows the temperatureprofile across the fuel cell stack at various times during thisconventional manner. Here, the ends of the stack cool more quickly thanthe cells in between and a convex shaped temperature profile isobserved.

The water distributions were markedly different between the two cooldownprocedures. Visually, the amount of liquid water on the anode platesessentially all seemed to have transferred to the cathode plates whenthe stack was cooled by the inventive method. However, when cooled viathe conventional method, the amount of water on each of the cathode andanode plates was highly variable. Some cathode plates had significantamounts of water while in other cells the anode plates had most of theliquid water. The MEAs obtained from cells cooled in the conventionalmanner had more water (from about 0.5 to 1.5 g) than MEAs from cellscooled using the inventive method (from about 0.4 to 1.1 g). However, inboth cases, this is a suitable amount of water for membrane electrolyteconductivity (the estimated maximum water content for the electrolyte ineach cell was about 0.6-0.8 g). Finally, as expected from the resultsfrom the whole MEAs, the membrane electrolytes within the MEAs containmore water after the conventional cooldown than their counterparts afterthe inventive cooldown. Further, liquid water was found in certain gasdiffusion layers following conventional cooldown but not following theinventive cooldown. As mentioned previously, the presence of liquidwater can pose a problem if the cell is to be stored below 0° C.

This Example shows that the advantages of the inventive method overconventional cooldown by desirably distributing water within a fuel cellstack.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from thespirit and scope of the present disclosure, particularly in light of theforegoing teachings.

1. A method of shutting down a fuel cell stack having at least two fuelcells stacked in series, each fuel cell having a cathode side and ananode side, the method comprising: stopping the generation ofelectricity from the stack; allowing the stack to cool over a cooldownperiod; and maintaining a temperature difference between the cathodeside and the anode side of each fuel cell during the cooldown period,wherein the direction of the temperature difference in each fuel cell isthe same.
 2. The method of claim 1 wherein the cathode side is hotterthan the anode side in each fuel cell during the cooldown period.
 3. Themethod of claim 1 wherein the anode side is hotter than the cathode sidein each fuel cell during the cooldown period.
 4. The method of claim 1wherein the stack is a solid polymer electrolyte fuel cell stack.
 5. Themethod of claim 1 wherein the stack comprises a plurality of fuel cellsstacked in series.
 6. The method of claim 1 wherein the temperatureprofile between the ends of the stack is sawtooth shaped during thecooldown period and wherein each tooth in the sawtooth shape correspondsto the temperature profile across a single fuel cell.
 7. The method ofclaim 6 wherein each fuel cell is thermally insulated from adjacent fuelcells in the stack.
 8. The method of claim 7 wherein the stack furthercomprises coolant channels having thermal insulating liners on one sideof the channels between adjacent fuel cells in the stack.
 9. The methodof claim 1 wherein the temperature profile between the ends of the stackis sawtooth shaped during the cooldown period and wherein each tooth inthe sawtooth shape corresponds to the temperature profile across a groupof fuel cells.
 10. The method of claim 9 wherein the stack furthercomprises Peltier devices between each group of fuel cells.
 11. Themethod of claim 1 wherein the stack further comprises a hot end and acold end and wherein the temperatures of the fuel cells decreasemonotonically between the hot end and the cold end during the cooldownperiod.
 12. The method of claim 11 wherein the hot end of the stack isthermally insulated.
 13. The method of claim 11 further comprisingheating the hot end of the stack.
 14. The method of claim 11 furthercomprising cooling the cold end of the stack.
 15. The method of claim 1wherein the temperature difference between the cathode side and theanode side of each fuel cell at the start of the cooldown period isgreater than 1° C. per fuel cell.
 16. The method of claim 1 wherein thetemperature of the stack prior to the cooldown period is greater thanabout 70° C.
 17. The method of claim 16 wherein the temperature of thestack after the cooldown period is less than about 40° C.
 18. The methodof claim 1 wherein each fuel cell comprises cathode and anode reactantflow fields and the reactant flow fields are not purged during thecooldown period.
 19. The method of claim 1 wherein each fuel cellcomprises cathode and anode reactant flow fields and the colder reactantflow field in each fuel cell is purged during the cooldown period.
 20. Afuel cell system comprising a fuel cell series stack having at least twofuel cells stacked in series, each fuel cell having a cathode side andan anode side, and means for shutting down the stack according to themethod of claim
 1. 21. The fuel cell system of claim 20 wherein thestack further comprises coolant channels having thermal insulatingliners on one side of the channels between adjacent fuel cells in thestack.
 22. The fuel cell system of claim 20 wherein the stack furthercomprises at least one Peltier device between a pair of cells in thestack.